Update metrics.rst (#551)

Fix error in docs where the line numbers are wrong for the binary accuracy example

docs/notes/metrics.rst

@@ -24,7 +24,7 @@ Metric Decorators
 Now that we have explained some of the basic aspects of the metric API, lets have a look at an example:
 
 .. literalinclude:: /../torchbearer/metrics/primitives.py
-   :lines: 17-22
+   :lines: 22-26
 
 This is the definition of the default accuracy metric in torchbearer, let's break it down.
 
@@ -86,4 +86,4 @@ When :meth:`.Metric.process` is called on a :class:`.MetricTree`, the output of
 of the children in turn. The results from the children are then collected in a dictionary. The main reason for including
 this was to enable encapsulation of the different statistics without each one needing to compute the underlying metric
 individually. In theory the :class:`.MetricTree` means that vastly complex metrics could be computed for specific use
-cases, although I can't think of any right now...
+cases, although I can't think of any right now...